System and method for testing a circuit

ABSTRACT

In one embodiment, a sensor for circuit testing has a first terminal and a second terminal. The first terminal is configured to be coupled to a first node of a first circuit via a first capacitor, and the second terminal is configured to be coupled to a second node of the first circuit. The sensor also has at least one transmitter and at least one receiver that measures a first transmission factor between the first terminal and the second terminal. The sensor determines that the first circuit is in a first state if the first transmission factor is above a first threshold, and determines that the first circuit is in a second state if the first transmission factor is below the first threshold.

TECHNICAL FIELD

This invention relates generally to electrical components, and moreparticularly to a system and method testing a circuit.

BACKGROUND

In many electronic applications, switches are used to control currentflow in a system. In high voltage systems, such as lighting systems andelectric machines, a multitude of switches can be used to enable anddisable high voltage and high current paths during system operation. Insafety-critical applications, such as elevators, and electricautomobiles, and public transportation vehicles, however, the state ofswitching components is tested and diagnosed in order to detect andremedy different types of system failures. For example, if a failedswitch is detected when an electric car is being turned on, a controlleron board the electric car can prevent the car from starting, well as loga message in the car's memory noting the switch failure so that thecar's manufacturer or mechanic can later be apprised of the source ofthe failure.

In systems where switches are connected in series, for example, inhigh-voltage systems, galvanic isolated circuitry is used to determinethe state of each single switch because the connecting points of theswitches can be at any potential in the system. Furthermore, isolateddiagnostic capability is used in cases where the power components andthe control components are not referring to the same ground potential,or are even isolated, such as the case in electric vehicles where theground used to reference the electric motor and other high voltagemachinery is isolated from the ground used to reference controlcircuitry and low power microelectronics.

In the case of high currents and/or voltages, mechanical switches, suchas relays, are used because their conduction losses are smaller than theconduction losses of semiconductor switches. To detect a switching statefor a relay, knowledge of whether the power contacts are open or closedis used. For example, in conventional isolated switch sensing devices,such as a force-guided contact relay, power contacts are mechanicallycoupled to a sensing contact that is opened and closed in parallel tothe power contact. In some cases, however, this mechanical couplingincreases the size and the complexity for manufacturing of a relay,especially if the relay is hermetically sealed for arc suppression.

In some systems, fuses are also used to control current flow. In theevent of an overload condition, a fuse is blown, which creates an opencircuit, thereby preventing further current flow. An overload conditionin the system can occur due to abnormal functioning of some components,or can occur due to other reasons, such as heavy external loading. If asystem is operated without regard to the state of a fuse, additionaldamage can be done to the system. What is needed are systems and methodsfor fuse and switch state diagnostics.

SUMMARY OF THE INVENTION

In one embodiment, a sensor for circuit testing has a first terminal anda second terminal. The first terminal is configured to be coupled to afirst node of a first circuit via a first capacitor, and the secondterminal is configured to be coupled to a second node of the firstcircuit. The sensor also has at least one transmitter and at least onereceiver that measures a first transmission factor between the firstterminal and the second terminal. The sensor determines that the firstcircuit is in a first state if the first transmission factor is above afirst threshold, and determines that the first circuit is in a secondstate if the first transmission factor is below the first threshold.

The foregoing has outlined, rather broadly, features of the presentinvention. Additional features of the invention will be described,hereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 a-1 b illustrate embodiment battery switch systems;

FIGS. 2 a-2 c illustrate embodiment topologies for switch statedetermination;

FIGS. 3 a-3 c illustrate further embodiment topologies for switch statedetermination;

FIGS. 4 a-4 b illustrate embodiment switch sensors that determine thestate of series connected switches;

FIG. 5 illustrates an embodiment switch sensor for a printed circuitboard (PCB);

FIG. 6 illustrates an embodiment switch measuring system;

FIG. 7 illustrates an embodiment switch measuring system;

FIG. 8 illustrates another embodiment switch measuring system;

FIG. 9 illustrates a further embodiment switch measuring system;

FIG. 10 illustrates a further embodiment switch measuring system;

FIG. 11 illustrates an embodiment transceiver (transmitter andreceiver); and

FIG. 12 illustrates an embodiment sensor system having multiple sensors.

Corresponding numerals and symbols in different figures generally referto corresponding parts unless otherwise indicated. The figures are drawnto clearly illustrate the relevant aspects of embodiments of the presentinvention and are not necessarily drawn to scale. To more clearlyillustrate certain embodiments, a letter indicating variations of thesame structure, material, or process step may follow a figure number.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of embodiments are discussed in detail below. Itshould be appreciated, however, that the present invention provides manyapplicable inventive concepts that may be embodied in a wide variety ofspecific contexts. The specific embodiments discussed are merelyillustrative of specific ways to make and use the invention, and do notlimit the scope of the invention.

The present invention will be described with respect to embodiments in aspecific context, namely systems and methods for testing switchingdevices in a circuit. Embodiments of this invention may also be appliedto systems and methods directed toward other forms of circuit testing.

FIG. 1 a illustrates an example embodiment of the present inventiondirected toward a battery system that has high voltage battery 110 inseries with fuse 118 and switches 120 and 122. Switches 120 and 122controlled by terminal pairs C+, C− and D+, D−, respectively, connectand disconnect connecting terminals HV+ and HV− from battery 110. Insome embodiments, switches 120 and 122 isolate battery 110 fromterminals HV+ and HV− when a connected load (not shown) is shut down ordisabled. In some embodiments, the load is shut down or disabled, forexample, when the system is undergoing maintenance. Fuse 118 breaks thecurrent conduction path of battery 110 in the case of a current overloadcondition. Sensor 102 monitors the state of fuse 118 via terminals 150and 152, sensor 104 monitors the state of switch 120 via terminals 154and 156, and sensor 106 monitors the state of sensor switch 122 viaterminals 158 and 160. In some embodiments, switches 120 and 122 areimplemented by single pole relays. Alternatively, other switch typessuch can be used.

FIG. 1 b illustrates embodiment battery system that connects battery 110to terminals HV+ and HV− via double pole switch 134 controlled byterminal pair E+, E−. Embodiment sensor 106 detects the state of theportion of switch 134 that connects battery 110 to HV− via terminals 158and 160. Embodiment sensor 132 detects the state of fuse 188 viaterminals 162 and 154, and detects the state of the portion of switch134 that connects battery 110 to terminal HV+ via terminals 164 and 166.In some embodiments, for example, where battery terminal connections HV+and HV− are in close proximity, double pole switch 134 can be used. Inother embodiments where terminal connections HV+ and HV− are not inclose proximity, independent switches 120 and 122 (FIG. 1 a) can beused.

In an embodiment of the present invention, a value of at least onecapacitance is modulated by the switch is sensed. If the switch isclosed, for example, the resulting capacitance seen at the input of theswitch is larger than the capacitance seen when the switch is open. Inembodiments, the changed capacitance is sensed by measuring an ACresponse though the switch.

FIGS. 2 a-c illustrate several embodiment topologies for switch statedetermination based on capacitive sensing. In FIG. 2 a, switch 208 ismonitored by sensor 202 via isolating capacitors 204 and 206. Switch 208represents any kind of switch, such as a simple mechanical switch, arelay, fuse, etc. Alternatively, switch 208 can be a junction (e.g. pnjunction) of a transistor or a FET channel, or any other componentmodulating a capacitance. In an embodiment, sensor 202 produces an ACtest signal at terminal 1 and receives the AC test signal at terminal 2.If the magnitude of the received test signal is greater than apredetermined threshold, switch 208 is determined to be closed. If, onthe hand, the magnitude is less than a predetermined threshold, switch208 is determined to be open. In an embodiment of the present invention,the AC signal is transmitted at an amplitude of about a few volts, witha frequency range of several hundred kHz to several MHz. Alternatively,other amplitudes and frequencies can be used. In an embodiment, anabsolute portion of a detected transmission factor from terminal 1 toterminal 2 is compared against a threshold that is defined by sensor202. In an embodiment, the threshold is predefined according to typicala application set stored in non-volatile memory locations (Flash,EEPROM, on-chip fuses, etc.) after a teach-in phase, or configured byexternal components (e.g. external R).

FIG. 2 b illustrates an embodiment of the present invention where switch208 is monitored by sensor 202. Sensor 202 is coupled to switch 208 viacapacitor 206 at terminal 2, and via a direct connection to switch 208at terminal 1. In embodiments where isolation between the sensor and themonitored device is not necessary, for example, where the sensor itselfhas an independent power supply and communication between the sensor andthe system control unit is already isolated, a direct connection can bemade to sensor 202.

FIG. 2 c illustrates an embodiment switch state sensor topology directedtoward a single pole double throw (SPDT) switch 226. Sensor 212 iscoupled to a common node of switch 226 via terminal 1. Sensor terminals3 and 2 are coupled to the remaining nodes of the switch via capacitors206 and 228 respectively. In an embodiment, an AC signal is transmittedfrom terminal 1 and received by terminals 3 and 2. If the magnitude ofthe received signal at terminal 2 is greater than the magnitude of thereceived signal at node 3, then the switch is determined to be in afirst state. If, on the other hand, the magnitude of the received signalat terminal 3 is greater than the magnitude of the received signal atnode 2, the switch is determined to be in the other state.Alternatively, the magnitudes of the received signals received at nodes3 and 2 can each be compared to a predetermined threshold to determineindependently whether either of the coupling paths are at a highimpedance state or a low impedance state, for example in cases where amultiplexer is monitored. FIG. 2 c illustrates terminal 1 being directlycoupled to switch 226. In alternative embodiments, terminal 1 can becoupled to switch 226 via a capacitor.

FIGS. 3 a-3 b illustrate several embodiment topologies for switch statedetermination based on capacitive sensing using an additional sensorreference terminal. FIG. 3 a illustrates sensor 302 coupled to switch308. Like the embodiments depicted in FIG. 2 a, terminals 1 and 2 ofsensor 302 are coupled to each node of switch 308 via capacitors 310 and314. In addition, sensor 302 has reference terminal R coupled to theleft side of switch 308 (same side as terminal 1). During operation,sensor 302 determines a first transmission factor between terminal 1 andterminal 2, and a second transmission factor between terminal 1 andterminal R. A transmission factor describes the relation between thetransmitted signal to the received signal and comprises informationabout attenuation (magnitude) and phase. If the ratio of the firsttransmission factor to the second transmission factor is greater than apredetermined threshold, switch 308 is determined to be closed. If, onthe other hand, the ratio of the first transmission factor to the secondtransmission factor is less than a predetermined threshold, switch 308is determined to be open. In an embodiment, reference terminal R andcapacitor 312 is used to compensate for drift effects of the circuit.Because a ratio of AC responses is used in determining the state ofswitch 308, the measurement threshold is not as sensitive to theabsolute values of the circuit elements, such as capacitors 310, 312 and314 or internal references. Measuring the ratio of AC responses alsocompensates for that drift effects in the sensing components and/orreferences, for example, due to aging, temperature, and supply voltage.

In some embodiments, drift effects are compensated when drift effectsare different for sensing elements and references. In a furtherembodiment, portions of the first transmission factor is comparedagainst portions of the second transmission factor. When the portionsshow similar drift behavior, less effort is required for compensation.

In embodiments of the present invention, the transmission factor can bemeasured in terms of voltage gain, current gain, tranconductance ortransresistance. For example, in an embodiment measuring thetransresistance, the transmitter transmits an AC current and thereceiver measures a received voltage. Alternatively, the capacitancebetween nodes can be measured using techniques described herein, as wellas techniques known in the art.

In an embodiment, capacitors 310, 312 and 314 are sized such that thetotal capacitance of capacitors 310 and 312 between terminals 1 and R islarger than the total capacitance of capacitors 310 and 314 betweenterminals 1 and 2 if the switch 308 is open, but is smaller if switch308 is closed. Alternatively, other ratios and relationships betweencapacitors can be used. FIG. 3 b, illustrates an embodiment in whichterminal 1 is directly coupled to switch 308.

FIG. 3 c illustrates an embodiment of the present invention where thestate of a SPDT switch or multiplexer is determined. SPDT switch 310 iscoupled to sensor 320 via capacitor 312 at terminal R, capacitor 314 atterminal 2, and capacitor 316 at terminal 3. Terminal 1 is shown with adirect connection to switch 310, however, in alternative embodiments,terminal 1 can be coupled to switch 310 via a capacitor. Duringoperation, a first AC magnitude response A₁₂ is measured betweenterminal 1 and terminal 2, a reference AC magnitude response A_(1R) ismeasured between terminals 1 and R, and a second AC magnitude responseA₁₃ is measured between terminals 1 and 3. Switch 310 is determined tobe in a first state if A₁₂/A_(1R) is greater than A₁₃/A_(1R). Likewise,switch 310 is determined to be in a second state if the ratio A₁₂/A_(1R)is less than A₁₃/A_(1R).

FIGS. 4 a and 4 b illustrate embodiment switch sensors that determinethe state of series connected switches. FIG. 4 a illustrates embodimentsensor 402 coupled to switches 404 and 406. Switch 406 is coupled tosensor 402 via capacitors 410, 412 and 414 to terminals 1, R and 2,respectively. Switch 404 is coupled to sensor 402 via capacitors 410,412 and 408 to terminals 1, R and 2′, respectively. Terminals 1 and Rare coupled to a common point between switches 404 and 406. Duringoperation, transmission factor A₁₂ is measured between terminal 1 andterminal 2, a transmission factorA_(1R) is measured between terminals 1and R, and transmission factorA_(12′), is measured between terminals 1and 2′. Switch 406 is determined to be opened if A₁₂/A_(1R) exceeds athreshold and closed if A₁₂/A_(1R) does not exceed the threshold.Likewise, switch 404 is determined to open if A_(12′)/A_(1R) exceeds athreshold and closed if A_(12′)/A_(1R) does not exceed the threshold. Inembodiments, measurements for A₁₂, A_(12′), and A_(1R) are performed inparallel using separate sensing circuits, serially using a singlesensing circuit, or a combination of both. In alternative embodiments ofthe present invention sensor 402 can be expanded to measure the morethan two switches sharing a common node coupled to terminals 1 and R.FIG. 4 b illustrates an embodiment switch sensor for series connectedswitches in which terminal 1 is directly coupled to a common pointbetween switch 404 and 406.

FIG. 5 illustrates an embodiment in which the state of a switch 504mounted on printed circuit board (PCB) 502 is measured by sensor 506.Terminals 1, 2 and R are coupled to switch 504 via the capacitance ofPCB 502. For example, electrode 512 and 514 coupled to terminals 1 andR, respectively, are disposed on one side of the PCB opposite trace 508coupled to one end of switch 504. Likewise, electrode 510 coupled toterminal 2 is also disposed on one side of the PCB opposite trace 516coupled to the other end of switch 504. The sensor operates as describedabove with respect to the embodiment of FIG. 3 a.

In an embodiment, electrical isolation between switch 504 and sensor 506is achieved by placing the components on a PCB (printed circuit board).If a minimum distance is kept between high-voltage parts and the sensor,isolation is ensured. The minimum distance depends on the isolationcapability of the material and the maximum voltage that should beblocked. Coupling elements are made by geometric overlap of high-voltageparts (shown here on the top side of the PCB) and the sensing elements(on the bottom side). The PCB itself ensures the galvanic isolation insome embodiments.

Due to small coupling effects between the overlapping areas on the topand on the bottom side of the PCB, and the resulting small capacitance,embodiment structures that use reference terminal R are advantageousbecause the ratio between two transmission factors are taken intoaccount, which compensates for drift effects. Furthermore, the verysmall coupling reduces the induced voltages in the sensor in the case ofswitching under load conditions or if a fuse is blown. In someembodiments, because of the small coupling, standard electro staticdischarge (ESD) protection elements of the sensor chip, such as thosefound on standard, inexpensive, low voltage semiconductor processes, arestrong enough to withstand a severe ESD event in the switch to bemeasured. Therefore, in some embodiments, no additional ESD protectionmeans are necessary in the sensor chip. In alternative embodiments,where the coupling is greater and/or the ESD events are more severe,additional ESD protection may be required.

In an embodiment of the present invention, determination of switchstates is based on the measurement of shift currents betweendielectrically isolated electrodes of the sensor arrangement andconducting portions of a switch being monitored. Electrodes arestimulated by an AC signal and shift currents are injected intoconnections on each side of a fuse, switch or relay. Shift currents cantake several return paths back to a stimulating source. These returnpaths are influenced by the switch position or fuse state. A closedswitch state introduces a low impedance conducting path between thecontacts of the monitored device, while an open switch offers a highimpedance and/or electrical isolation, therefore, an expected change inthe return paths of the shift currents is detectable. Here, thetransmission factor of the paths differ and the differences can be usedto detect the state of a switch. In some embodiments, a shift in returnpaths is measured in the stimulation path that is connected to aninjection electrode, which is an electrode connected to an AC signalsource, or is detected using an additional receiver coupled to a pointon the other side of the switch. In an embodiment, the coupling of thereceiver to the circuit is realized by a direct galvanical contact orcapacitively by a second electrode. Depending on the amplitude of theshift current, a determination is made of whether the switch is openedor closed.

FIG. 6 illustrates embodiment measurement system 600. Sensor IC 602 hasreceiver 604 coupled to a first side of switch 620 via capacitor CC1,and transmitter 606 coupled to a second side of switch 620 via capacitorCC2. Switch 620 couples Circuit 1 to Circuit 2. In some embodiments,Circuit 1 can be a power supply and Circuit 2 can be a load. ImpedanceZL1 represents the impedance between Circuit 1 and switch 620, impedanceZL2 represents the impedance between switch 620 and circuit 2.Impedances ZL1′, ZL2′ and ZL3 represent the ground impedances fromCircuit 1, Circuit 2 and IC 602 to GND, respectively.

According to FIG. 6, there are three return paths for a currentgenerated at transmitter:

1. CC2→Switch 620→CC1→Receiver 604;

2. CC2→Switch 620→ZL1→Circuit 1→ZL1′→ZL3; and

3. CC2→ZL2→Circuit 2→ZL2′→ZL3.

When switch 620 is closed and the impedance of path 1 is low compared topath 2 and path 3 in parallel, it is assumed that a detectable currentcan be measured at receiver 604. This condition is fulfilled if an ACsignal is chosen appropriately with respect to frequency and amplitude.Further, the impedance of paths 2 and 3 can be adjusted by introductionof the impedance ZL3, if necessary. When switch 620 is open, currentalong path 1 is due to parasitic capacitive coupling between thedisconnected parts of switch 620, thereby reducing the coupling betweenCC1 and CC2. In embodiments, the current in the parasitic return pathwhen the switch is open is less than the current in path 1 when theswitch is closed. Differences in measured currents can be directlyrelated to the state of the switch and is usable for monitoring of theswitch in some embodiments. In one embodiment, switch states aredetermined by comparing the measurement to a threshold. For example, ifthe measured current through path 1 is above the threshold, switch 620is determined to be open. If, on the other hand the current through path1 is below the threshold, switch 620 is determined to be open.

In one embodiment, the impedances of circuit1, circuit2 and the otherZLs are not considered (see FIG. 6). At least one side of the switch isdecoupled by a capacitor because the voltage between the contacts of aswitch can easily exceed the voltage capability of the sensor device,especially when opening the switch under inductive load conditions dueto wiring.

FIG. 7 illustrates embodiment measurement system 700. Sensor IC 702 hasreceiver 706 coupled to a first side of switch 620 via capacitor CC1,transmitter 708 coupled to the first side of switch 620 via capacitorCC3, and transmitter 704 coupled to a second side of switch 620 viacapacitor CC2. Switch 620 couples Circuit 1 to Circuit 2. In someembodiments, Circuit 1 can be a power supply and Circuit 2 can be aload. Impedance ZL1 represents the impedance between Circuit 1 andswitch 620, impedance ZL2 represents the impedance between switch 620and circuit 2. Impedances ZL1′, ZL2′ and ZL3 represent the groundimpedances from Circuit 1, Circuit 2 and IC 702 to GND, respectively.

In the embodiment shown in FIG. 7, IC 702 determines the decisionthreshold using a reference measurement, thereby making thedetermination of the state of switch 620 independent of the fabricationspread of CC1 and CC2. In this case, CC3 is chosen to be smaller thanCC2, which can be accomplished, for example, by scaling the electrodearea in PCB implementations. This creates a condition where the currentmeasured at receiver 706, during stimulation with the transmitter 704when the switch is closed, is larger then the current measured duringtransmission with transmitter 708. In this case, the switch isdetermined to be closed if the current measured from transmitter 704 ishigher than a corresponding measurement using transmitter 708. If thisis not the case, switch 620 is determined to be open.

In an alternative embodiment, capacitances CC1, CC2 and CC3 are scaledby changing the distance between the coupling electrodes at bothtransmitters instead adjusting the area of the electrodes. In this casethe distance between the switch current path and the electrode attransmitter 704 is lower than the distance of the electrode attransmitter 708. In a further embodiment, scaling is achieved by scalingthe stimulus voltages output from transmitters 704 and 708 instead ofadjusting the geometries of capacitors CC1, CC2 and CC3. Alternatively,a combination of geometric and voltage scaling methods can be used.

FIG. 8 illustrates embodiment measurement system 800, which is similarto embodiment 700 of FIG. 7 with the addition of closed dummy switch 810coupled in series with capacitor CC3. The resistance of dummy switch 810compensates for resistance in switch 620. For example, in embodimentswhere the switch resistance of switch 620 is non-negligible, the highswitch resistance may effect the measured ratio of the transmittedcurrent though CC2 and the transmitted current though CC3. In someembodiments, dummy switch 810 has similar impedance characteristics asswitch 620, or is made from a same type of switch. In other embodiments,dummy resistor 810 can be implemented using a resistor. In furtherembodiments, dummy resistor 810 is greater than the maximum resistors ofswitch 620.

FIG. 9 illustrates alternative embodiment measurement system 810, whichis similar to embodiment 800 of FIG. 7, except that the transmit andreceive directions have been swapped. Instead of having two transmittersand one receiver, IC 802 has a single transmitter 806 coupled to CC1,and two receivers 804 and 808 coupled to CC2 and CC3 respectively. Byhaving two receivers, both measurements though CC2 and CC3 can beperformed simultaneously, thereby reducing detection time in someembodiments.

FIG. 10 illustrates alternative embodiment measurement system 860, whichis similar to embodiment 700 of FIG. 7, except that capacitors CC1, CC2,CC3, and dummy switch 810 are disposed on IC 820. In an alternativeembodiment, dummy switch 810 is optional and can be omitted, especiallyin cases where the closed switched resistance of switch 620 is verysmall.

FIG. 11 illustrates embodiment transceiver 900 that transmits signalsSend 1 and Send 2, and receives signal Receive. The transmission pathincludes clock generator 910 coupled to pattern generator 908. Patterngenerator 908 divides a clock signal provided by clock 910 and providesa pattern signal to edge control circuit 906, which provides slew ratecontrol to current sources 902 and 904 in order to create a trapezoidalsignal on signals Send 1 and Send 2. The receive path includes amplifier918 followed by processor 920. Optional receive switch matrix 914 andphase adaptor 912 are used in an embodiment synchronous demodulationscheme.

In an embodiment trapezoid, signals have controlled rise and fall times.A multiplexed transmitter output can be achieved by switching a currentsource for charging or discharging the selected output via switch matrix916. In an embodiment, the receiver uses a synchronous demodulationscheme based on a same pattern that is applied to control the transmitsequence. The phases of the pattern can be adapted to different delaysthat are caused by the different impedances of the different measurementpaths in order to generate a demodulated signal with maximum signalenergy. The demodulation itself is done by cross switching the incomingmeasurement current between the inverting and non inverting input of thefollowing signal processing chain using switch matrix 914. In anembodiment, switch matrix 914 uses synchronous rectification. Thedemodulated current is converted into a voltage and subsequentlyamplified, filtered and A/D converted, in some embodiments.Alternatively, the A/D can be omitted and the switch determination madevia analog processing. Finally a decision on the state of the monitoredswitch is taken and passed to the interface represented by signalOutput.

FIG. 12 illustrates an embodiment system in which an arbitrary number ofsensors 930, 932 and 934 are coupled to system controller 942 via timingmechanism and interface 940. Sensors 930, 932 and 934 are implementedaccording to embodiments described herein. To minimize power consumptionand interference between sensors 930, 932 and 934, not all sensors areactivated at the same time. Timing mechanism and interface 940,controlled by system controller 942, activates and receives the resultsfrom sensors 930, 932 and 934. In one embodiment, timing mechanism andinterface 940 is configured to activate one sensor at a time in around-robin fashion. In alternative embodiments of the presentinvention, a subset or a limited number of the sensors are activated atthe same time, as defined by the timing mechanism.

In an embodiment, delay information can be used as an additional inputfor switch monitoring if the current path has a different delaycharacteristic in case of the open and the closed switch, for example,in cases where signal paths via the supply source or the connected loadinclude long lines or coils. Alternatively, I/Q demodulation can be usedto get additional information about the absolute value and the phaserelation of the incoming current and the transmitted sequence. The I/Qdemodulation replaces the adaptation of the delay time for thedemodulation.

Advantages of some embodiments of the present invention includedetection of a switching state independent from the voltage level at theswitch contacts. Certain embodiment sensors, therefore, are operablewith switches at any voltage potential in a system. Furthermore, someembodiments of the present invention optionally offer galvanic isolationof the detection circuit from the power circuit for the case where thepower side of a system and the control side are not referenced to thesame ground potential. In some advantageous embodiments, the switchsensor is not sensitive to voltage levels at the power contacts of theswitch, for example, in the case of high static voltage and fasttransients. In some embodiments, advantages of capacitive couplingbetween the sensor and contacts of the switch being evaluated arerelated to the voltage across an open switch, in that a small couplingcapacitance limits the voltage over the sensor, even if the switch workswith high voltages.

Another advantage of some embodiments is that each switch or fuse in amulti-switch system can be analyzed independently from other switchesand fuses in the system.

In some advantageous embodiments, the switch detection mechanism isoperable both with and without current flowing through the switch orfuse. Furthermore, in embodiments where there is no parasitic currentpath from the switch sensor, the sensor can remain operable under loadconditions or overload conditions. Furthermore, embodiment sensors canbe operated while the system is running, as well as when portions of thesystem are powered down, or even disconnected from the power supply. Insome advantageous embodiments, no significant current is induced in thepower switch by the diagnostic means.

In some embodiments, where there is slow switching capability of relaysor fuses, the switch sensor can also operate slowly.

Another advantage of some embodiments is low production costs. Forexample, costs are saved in embodiments that do not require trimming.Furthermore, lower bill of material costs are achieved, for example, inembodiments that do not use high voltage components within the switchsensor, because inexpensive, low voltage semiconductor processes can beused to fabricate the sensors.

It will also be readily understood by those skilled in the art thatmaterials and methods may be varied while remaining within the scope ofthe present invention. It is also appreciated that the present inventionprovides many applicable inventive concepts other than the specificcontexts used to illustrate embodiments. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A sensor for circuit testing, the sensor comprising: a first terminalconfigured to be coupled to a first node of a first circuit via a firstcapacitor; a second terminal configured to be coupled to a second nodeof the first circuit; at least one transmitter and at least one receivermeasuring a first transmission factor between the first terminal and thesecond terminal, wherein the sensor determines that the first circuit isin a first state if the first transmission factor is above a threshold;and the sensor determines that the first circuit is in a second state ifthe first transmission factor is below the threshold.
 2. The sensor ofclaim 1, wherein the second terminal is configured to be coupled to thesecond node of the first circuit via a second capacitor.
 3. The sensorof claim 1, wherein the at least one transmitter is coupled to the firstterminal and the at least one receiver is coupled to the secondterminal.
 4. The sensor of claim 1, wherein: the circuit comprises aswitch; the switch is closed in the first state; and the switch is openin the second state.
 5. A sensor for circuit testing, the sensorcomprising: a first terminal configured to be coupled to a first node ofa first circuit via a first capacitor; a second terminal configured tobe coupled to a second node of the first circuit; a third terminalconfigured to be coupled to the second node of the first circuit via athird capacitor at least one transmitter and at least one receivermeasuring a first transmission factor between the first terminal and thesecond terminal, wherein the at least one transmitter and the at leastone receiver further measures a second transmission factor between thethird terminal and the second terminal; the sensor further determinesthat the first circuit is in a first state if a ratio of the firsttransmission factor to the second transmission factor is above a firstthreshold; and the sensor further determines that the first circuit isin a second state if the ratio of the first transmission factor to thesecond transmission factor is below the first threshold.
 6. The sensorof claim 5, wherein the second terminal is configured to be coupled tothe second node of the first circuit via a second capacitor.
 7. Thesensor of claim 5, wherein a first of the at least one transmitter iscoupled to the first terminal; a second of the at least one transmitteris coupled to the third terminal; the at least one receiver is coupledto the second terminal.
 8. The sensor of claim 5, wherein a first of theat least one receiver is coupled to the first terminal; a second of theat least one receiver is coupled to the third terminal; and the at leastone transmitter is coupled to the second terminal.
 9. The sensor ofclaim 7, wherein the sensor is disposed on an integrated circuit. 10.The sensor of claim 9, wherein at least one capacitor is disposed on theintegrated circuit.
 11. The sensor of claim 5, wherein: the circuitcomprises a switch; the switch is closed in the first state; and theswitch is open in the second state.
 12. A system for in-situ testing ofswitches, the system comprising a sensor, the sensor comprising a firstterminal configured to be coupled to a first node of a first switch; asecond terminal configured to be coupled a second node of the firstswitch; a third terminal configured to be coupled to the second node ofthe first switch; and at least one transmitter and at least one receivermeasuring a first transmission factor between the first terminal and thesecond terminal, the at least one transmitter and the at least onereceiver further measuring a second transmission factor between thethird terminal and the second terminal, wherein the sensor determinesthat the first switch is closed if a ratio of the first transmissionfactor to the second transmission factor is above a first threshold; andthe sensor determines that the first switch open if the ratio of thefirst transmission factor to the second transmission factor is below thefirst threshold.
 13. The system of claim 12, further comprising: a firstcapacitor coupled between the first terminal of the sensor and the firstnode of the first switch; and a third capacitor coupled between thethird terminal of the sensor and the second node of the first switch.14. The system of claim 13, wherein: at least one capacitor comprises afirst electrode coupled to one node of the first switch via a dielectriccomprising printed circuit board (PCB) material.
 15. The system of claim13, further comprising a dummy switch coupled in series with the thirdcapacitor.
 16. The system of claim 15, wherein the dummy switchapproximates an impedance of the first switch when the first switch isclosed.
 17. The system of claim 13, further comprising a secondcapacitor coupled between the second terminal of the sensor and thesecond node of the first switch.
 18. The system of claim 12, wherein:the sensor further comprises a fourth terminal configured to be coupledto a first node of a second switch, and the third terminal is furtherconfigured to be coupled to a second node of the second switch; the atleast one transmitter and the at least one receiver further measures athird transmission factor between the fourth terminal and the secondterminal; the sensor further determines that the second switch is closedif a ratio of the third transmission factor to the second transmissionfactor is above a second threshold; and the sensor further determinesthat the first switch open if the ratio of the third transmission factorto the second transmission factor is below the second threshold.
 19. Thesystem of claim 18, further comprising: a first capacitor coupledbetween the first terminal of the sensor and the first node of the firstswitch; a second capacitor coupled between the second terminal of thesensor and the second node of the first switch; a third capacitorcoupled between the third terminal of the sensor and the second node ofthe first switch; and a fourth capacitor coupled between the fourthterminal of the sensor and the first node of the second switch.
 20. Thesystem of claim 12, further comprising: a plurality sensors configuredto coupled to a plurality of switches; and a controller coupled to theplurality of sensors, the controller configured to activate theplurality of sensors a subset at a time.
 21. A method for testing atleast one switch, the method comprising: measuring a first ACtransmission factor through a first signal path, the first signal pathcomprising a first series capacitor and a first terminal and a secondterminal of the at least one switch; measuring a second AC transmissionfactor though a second signal path, the second signal path comprisingthe first terminal of the at least one switch and a second seriescapacitor; determining that the at least one switch is closed if a ratioof the first AC transmission factor to the second AC transmission factoris greater than a threshold; determining that the at least one switch isclosed if the ratio of the first AC transmission factor to the second ACtransmission factor is less than the threshold.
 22. The method of claim21, further comprising determining the threshold, comprising: measuringa first ratio of the first AC transmission to the second AC transmissionwhen the at least one switch is open; measuring a second ratio of thefirst AC transmission to the second AC transmission when the at leastone switch is closed; determining the threshold to be between the firstratio and the second ratio.
 23. The method of claim 21, wherein thefirst and second signal paths further comprise a third series capacitor.24. The method of claim 23, wherein: measuring the first AC transmissionfactor comprises: transmitting an AC test signal to the first terminalof the at least one switch, receiving the AC test signal from the secondterminal of the at least one switch via the first series capacitor; andmeasuring the second AC transmission factor comprises: transmitting anAC test signal to the first terminal of the at least one switch,receiving the AC test signal from the first terminal of the at least oneswitch via the second series capacitor.
 25. The method of claim 24,wherein: transmitting the AC test signal to the first terminal comprisestransmitting a current; and receiving the AC test signal from the secondterminal comprises receiving a voltage.